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The Last Word

3 April 1999

Biosphere

Question: Hypothetically (because otherwise my mum would get mad), if I were
to put my brother in a perfectly sealed room, how much plant life would I need
in that room in order to maintain a balance of oxygen and carbon dioxide such
that both my brother and my beloved plants continue to live?

Answer: To simplify matters, you could supply his meal through an airtight
hatch. The plants would then only need to provide his oxygen. If he spent all
his time eating and dozing, he would need about 350 litres of oxygen per day
(the amount of oxygen in 1.7 cubic metres of air). This much oxygen is produced
in full sunlight by typical vegetation covering a floor area of between 5 and 20
square metres. Using the most productive “C4 plants” such as sugar
cane, you could reduce the area needed to 2.5 square metres. Your brother would
exhale 350 litres of carbon dioxide per day, which would enable the plants to
grow with an increase of dry weight of 430 grams per day.

Now let’s muddy the waters. If his windows plus artificial lights supply 10
per cent of full sunlight, multiply the required area of greenery by a factor of
10. If the lights go out at night, double the area—more in winter. Plants
photosynthesise during the day more rapidly than they respire at night.
Therefore, as a reasonable approximation, you can neglect the extra oxygen that
plants consume at night.

If you don’t intend to feed your brother, but hope he will survive by eating
the plants, remember most material a plant synthesises is indigestible, so
double the area again. The inedible parts of the plants plus your brother’s
faeces would need to be decomposed or burnt to carbon dioxide to recycle the
carbon they contain. So, if your brother is a well-trained plant physiologist,
this ambitious biosphere might need to be a plant-filled room about 20 metres
square.

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Basis of calculations:

Daily energy requirement of an adult dozing is 1750 kilocalories per day.
Energy content of 100 grams of sucrose is 400 kilocalories. Therefore an adult
needs 1750/400 = 438 grams of sucrose per day = 1.28 moles sucrose per day.

Photosynthesis rates of plants in the field under optimal lighting are
between 10 and 30 micromoles (up to 70 in C4 plants) of carbon dioxide
fixed per square metre per second (0.86 to 6.05 moles per square metre per day).
For each mole of carbon dioxide the plants fix, they liberate a mole of
oxygen.

Therefore the area required is somewhere between 18 square metres for the
less productive plants down to 2.5 square metres for C4 plants.

Stephen Fry

Institute of Cell and Molecular Biology, Edinburgh University

Answer: This is an important question for our future in space. The first
experiments with uncrewed ecosystems were performed by Russian scientists in the
1950s. This led to the crewed closed facility Bios-3 in 1965, a 315-cubic-metre
habitat at the Institute of Biophysics in Krasnoyarsk, Siberia.
Chlorella algae, which photosynthesise, were used to recycle air breathed
by humans, absorbing carbon dioxide and replenishing it with oxygen. The algae
were cultivated under artificial light. To achieve a balance of oxygen and
carbon dioxide, one human needed 8 square metres of exposed Chlorella.
The algae tanks were stacked so they took less than 8 square metres floor space.
Water and nutrients were stored in advance—these were recycled too. By
1968, the system efficiency had reached 85 per cent by recycling water. Bios-3
has conducted tests with two and three people for up to six months.

NASA is also conducting Controlled Ecological Life Support Systems
experiments. One of its core systems is the Biomass Production Chamber, a
sealable steel chamber about 3.5 metres in diameter and 7.5 metres high with a
plant-growing area of 20 square metres. In 1989, NASA completed BioHome, which
integrated biogenerative components for recycling air, water and nutrients from
human waste into a single habitat.

Rudy Vaas

Bietigheim-Bissingen, Germany

Irrational breakdown

Question: Why is it that only microorganisms break down things like
cellulose, lignin and beeswax? Presumably it is just a question of producing the
right enzymes—yet humans can’t do it, and even cows rely on bacteria in
their gut to digest their plant food for them. What a colossal selective
advantage it would offer, so why has evolution so far excluded it?

Answer: Animals evolved on or in marine sediments, feeding on bacteria,
protists and on one another. They did not need enzymes to digest algal and plant
material. Modern animals bear the consequences of this evolution. If they are to
exploit plants as food, they must use symbiotic microbes, or rely on an
evolutionary “reinvention” of cellulase and other enzymes. The latter has
happened in some animal groups, notably snails and their relatives. However, it
is more common to use microbes living in a specialised part of the gut: examples
range from cows to termites.

This has arisen because the evolution of a suite of enzymes which can handle
cell wall materials is a slow and uncertain business, relying on multiple
mutations and therefore on chance. Equivalent enzymes are there to be harnessed,
now, in microbes. Secondly, the disadvantages of using symbiotic microbes are
not great. The unicells can themselves be used as food by the animal. Some
energy will be lost as heat through microbial metabolism, but even this has
benefits in cold climates. Thirdly, plant material typically contains little
protein; wood is especially nitrogen-poor.

A bacterial community can improve things. Ruminants, such as the cow, can
secrete urea into the rumen. The microbes can use this nitrogen source,
otherwise excreted, and incorporate it digestible into protein.

Similar arguments apply to wax. Interestingly, wax is very important as food,
globally. Waxes are important energy stores in copepods—crustaceans that
play a role in planktonic food chains. Many fish and seabirds can digest waxes,
using enzymes they make themselves or from symbiotic microbes.

Julian Sutton

Totnes, Devon

There may also be a more general reason why not all species can do t
he same things. Scientists studying scaling laws (“Ruling Passions” p 34) have
shown there are fundamental principles behind the great differences in energy
efficiency between small and large organisms, which will in turn make different
food sources more or less usable—Ed